One problem which often occurs in radio transmissions over a channel is that a transmitted signal is subjected to multipath propagation and noise. In the case of mobile telephony for instance, the transmission properties of the channel will shift due to a change in the mutual positions of the transmitter and receiver. These problems have been solved in time-shared, digital radio transmission systems, in that the time-slot transmitted signal sequences include a synchronizing sequence and a data sequence. The synchronizing sequence is known to the receiver and the receiver is able to evaluate the transmission properties of the channel, i.e. effect a channel estimate, on the basis of this sequence. With the aid of this channel estimate, the receiver is able to evaluate the symbols of the data sequence which contains the information to be transmitted.
However, in certain instances it has been found insufficient to effect a channel estimate only once with each time slot. In the case of time slots of long duration, i.e. in the order of several milliseconds, the transmitter and receiver have sufficient time to change their mutual positions considerably during the course of a time slot. Consequently, the transmission properties of the channel can change radically during the duration of the time slot, so that the receiver estimation of the transmitted symbols becomes deficient and the transmitted information contains interferences. A radio receiver in which these interferences are partially avoided is found described in an article in IEEE Transactions On Information Theory, January 1973, pages 120-124, F. R. Magee Jr and J. G. Proakis: "Adaptive Maximum-Likelihood Sequence Estimation for Digital Signaling in the Presence of Intersymbol Interference". The article describes an equalizer which includes a viterbianalyzer equipped with an adaptation filter as a channel estimating circuit.
The equalizer described in this article partially overcomes those problems which occur with long time slots, although it has the disadvantage of lacking the ability to perform a correct adaptation after fading, during which the signal strength falls beneath the noise level. After fading has taken place the equalizer has difficulty in re-adapting to the data sequence, which is a sequence unknown to the receiver.
Fading occurs as a result of signal interference between signals reflected along mutually separate paths, such that fading often recurs for a mobile receiver which moves in the interference pattern of the signals. This can result in a large proportion of transmitted signal sequences being subjected to fading, so that a large part of the transmitted information will be lost. A solution to the problem of adapting to an unknown signal sequence is given in an article in IEEE Transactions on Communications, Vol Com-28, No. 11, November 1980, D. N. Godard: "Self-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communication Systems". Equalizer adaptation in the event of intersymbol interference is achieved by the introduction of an algorithm with a new type of cost functions and by minimizing these functions. The algorithm, however, converges relatively slowly and cannot be utilized when desiring to adapt an equalizer during one of the aforesaid time slots with a duration in the order of milliseconds.